Flash memory devices and methods for fabricating the same

ABSTRACT

Flash memory devices and methods for fabricating the same are provided. In accordance with an exemplary embodiment of the invention, a method for fabricating a memory device comprises the steps of fabricating a first gate stack and a second gate stack overlying a substrate. A trench is etched into the substrate between the first gate stack and the second gate stack and a first impurity doped region is formed within the substrate underlying the trench. The trench is filled at least partially with a conductive material.

FIELD OF THE INVENTION

The present invention generally relates to flash memory devices, andmore particularly relates to memory devices with reduced transientprogram disturb and methods for fabricating the same.

BACKGROUND OF THE INVENTION

A type of commercially available flash memory product is a MirrorBit®memory device available from Spansion, LLC, located in Sunnyvale, Calif.A MirrorBit cell effectively doubles the intrinsic density of a flashmemory array by storing two physically distinct bits on opposite sidesof a memory cell. Each bit within a cell can be programmed with a binaryunit of data (either a logic one or zero) that is mapped directly to thememory array.

An exemplary MirrorBit® memory device 10, illustrated in FIG. 1,includes a P-type semiconductor substrate 12 within which are formedspaced-apart source/drain regions 14, 16 respectively (both typicallyhaving N-type conductivity), otherwise known as bit lines. A chargetrapping layer or stack 18 is disposed on the top surface of thesubstrate between the bit lines. The charge trapping stack 18 typicallycomprises, for example, a charge trapping layer, often a silicon nitridelayer 20, disposed between a first or bottom silicon dioxide layer(commonly referred to as a tunnel oxide layer) 22 and a second or topsilicon dioxide layer 24. A gate electrode 26, which typically comprisesan N or N+ polycrystalline silicon layer, is formed over the chargetrapping stack to form a first storage element or bit 28 and a secondstorage element or bit 30 of memory cells 32 and 34. The charge trappinglayer 20 of first storage bit 28 and the charge trapping layer 20 ofsecond storage bit 30 of each memory cell 32 and 34 can be integral, asillustrated in FIG. 1, or can be separated by an isolation element,which typically is an oxide often referred to as a gate oxide.

A dual bit memory cell 34 is programmed utilizing a hot electroninjection technique. More specifically, programming of the first bit 28of memory cell 34 comprises injecting electrons into the charge trappinglayer 20 and applying a bias between bit lines 14 and 16 while applyinga high voltage to the control gate 26. In an exemplary embodiment, thismay be accomplished by grounding bit line 16 and applying approximately5 V to bit line 14 and approximately 10 V to the control gate 26. Thevoltage on the control gate 26 inverts a channel region 36 while thebias accelerates electrons from bit line 14 into the channel region 36towards bit line 16. The 4.5 eV to 5 eV kinetic energy gain of theelectrons is more than sufficient to surmount the 3.1 eV to 3.5 eVenergy barrier at channel region 36/tunnel oxide layer 22 interface and,while the electrons are accelerated towards source/drain region 16, thefield caused by the high voltage on control gate 26 redirects theelectrons towards the charge trapping layer of first bit 28. Thoseelectrons that cross the interface into the charge trapping layer remaintrapped for later reading.

Similarly, programming the second bit 30 by hot electron injection intothe charge trapping layer 20 comprises applying a bias between bit lines16 and 14 while applying a high voltage to the control gate 26. This maybe accomplished by grounding bit line 14 and applying approximately 5Vto bit line 16 and approximately 10 V to the control gate 26. Thevoltage on the control gate 26 inverts the channel region 36 while thebias accelerates electrons from bit line 16 into the channel region 36towards bit line 14. The field caused by the high voltage on controlgate 26 redirects the electrons towards the charge trapping layer ofsecond bit 30. Those electrons that cross the interface into chargetrapping layer 20 of second bit 30 remain trapped for later reading.

With advances in semiconductor process technology, the trend is towardsmaller and smaller semiconductor devices, including memory devices.However, as the—above described MirrorBit® is scaled smaller in size,transient program disturb (TPD) becomes a bigger challenge. TPD resultsduring programming of the dual bit devices. For example, hot holesgenerated during programming of first bit 28 of memory cell 32 have asecondary impact ionization below bit line 16. Secondary electronsresulting from the secondary impact ionization diffuse to the disturbedcell 34 below gate 26 where they are accelerated by the drain depletionregion and injected into the charge trapping stack 18 of second bit 30.This injection of secondary electrons into second bit 30 of cell 34 canadversely affect the memory window of bit 30.

Accordingly, it is desirable to provide a flash memory device withreduced transient program disturb. In addition, it is desirable toprovide methods for fabricating flash memory devices with reducedtransient program disturb. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description of the invention and the appendedclaims, taken in conjunction with the accompanying drawings and thisbackground of the invention.

BRIEF SUMMARY OF THE INVENTION

In accordance with an exemplary embodiment of the present invention, amethod for fabricating a memory device comprises the steps offabricating a first gate stack and a second gate stack overlying asubstrate. A trench is etched into the substrate between the first gatestack and the second gate stack and a first impurity doped region isformed within the substrate underlying the trench. The trench is filledat least partially with a conductive material.

In accordance with another exemplary embodiment of the presentinvention, a method for fabricating a dual bit memory device comprisesthe steps of fabricating a charge trapping stack overlying a substrateand forming a control gate material overlying the charge trapping stack.The control gate material and the charge trapping stack are etched tothe substrate to form a plurality of first gate stacks and a pluralityof second gate stacks. The plurality of first gate stacks are disposedin a linear manner along a length of the substrate and the plurality ofsecond gate stacks are disposed in a linear manner along the length ofthe substrate parallel to the plurality of first gate stacks. Thesubstrate is etched to form a trench that extends between the pluralityof first gate stacks and the plurality of second gate stacks along thelength of the substrate. A first impurity dopant is implanted within thesubstrate underlying the trench to form a first bit line region and aconductor is formed within the trench extending along the length of thesubstrate.

In accordance with a further exemplary embodiment of the presentinvention, a method for fabricating a memory device comprises the stepsof fabricating a first gate stack and a second gate stack overlying asubstrate and forming a first impurity doped region in the substratebetween the first gate stack and the second gate stack. Gate spacers areformed about sidewalls of the first gate stack and the second gate stackand a second impurity doped region is formed in the substrate betweenthe first gate stack and the second gate stack using the gate spacers asan implant mask. The second impurity doped region is deeper than thefirst impurity doped region. A space between the first gate stack andthe second gate stack is filled with a conductive material.

In accordance with yet another exemplary embodiment of the presentinvention, a memory device comprises a plurality of first gate stacksand a plurality of second gate stacks overlying a substrate. Theplurality of first gate stacks are disposed in a linear manner along alength of the substrate and the plurality of second gate stacks aredisposed in a linear manner along the length of the substrate parallelto the plurality of first gate stacks. A conductor is disposed within atrench in the substrate between the plurality of first gate stacks andthe plurality of second gate stacks. The conductor and the trench extendalong the length of the substrate. A first impurity doped region isdisposed within the substrate and underlying the trench and theconductor. The first impurity doped region extends along the length ofsubstrate.

In accordance with a further exemplary embodiment of the presentinvention, a memory device comprises a plurality of first gate stacksand a plurality of second gate stacks overlying a substrate. Theplurality of first gate stacks are disposed in a linear manner along alength of the substrate and the plurality of second gate stacks aredisposed in a linear manner along the length of the substrate parallelto the plurality of first gate stacks. A first impurity doped region isdisposed within the substrate between the plurality of first gate stacksand the plurality of second gate stacks along the length of substrate. Asecond impurity doped region is disposed within the substrate betweenthe plurality of first gate stacks and the plurality of second gatestacks along the length of substrate. The second impurity doped regionis disposed deeper within the substrate than the first impurity dopedregion. A conductor is disposed overlying the substrate between theplurality of first gate stacks and a plurality of second gate stacks.The conductor extends along the length of the substrate and is inelectrical contact with the first and second impurity doped regions.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a cross-sectional view of a portion of a dual bit memorydevice available in the prior art;

FIG. 2 is a cross-sectional view of a portion of a dual bit memorydevice in accordance with an exemplary embodiment of the presentinvention;

FIG. 3 is a top view of a portion of the dual bit memory device of FIG.2;

FIGS. 4-7 illustrate, in accordance with an exemplary embodiment of thepresent invention, a method for fabricating a portion of the dual bitmemory device of FIG. 2;

FIGS. 8-12 illustrate a method for fabricating a portion of a dual bitmemory device in accordance with another exemplary embodiment of thepresent invention;

FIGS. 13-17 illustrate a method for fabricating a portion of a dual bitmemory device in accordance with a further exemplary embodiment of thepresent invention; and

FIGS. 18-21 illustrate a method for fabricating a portion of a dual bitmemory device in accordance with another exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

Referring to FIG. 2, in accordance with an exemplary embodiment of thepresent invention, a portion of a dual bit flash memory device 50 thatmay be programmed with reduced transient program disturb challenges isillustrated. Memory device 50 has a first gate stack 52 and a secondgate stack 54 disposed on a P-type silicon substrate 56. Each gate stack52 and 54 comprises a multi-layer dielectric-charge trapping-dielectricstack 58. Charge trapping stack 58 may comprise a thin first insulatinglayer 64, a charge trapping layer 60, and a second insulating layer 62.Thin first insulating layer 64 is often referred to as a tunnel oxidelayer, a layer through which programming or erasing charge carriers cantunnel. Charge trapping layer 60 can be, for example, a layer of siliconnitride, silicon-rich silicon nitride, polycrystalline silicon, acombination of these, or any of the other well known charge trappingmaterials. Stoichiometric silicon nitride is Si_(x)N_(y) for which x=3and n=4; silicon-rich silicon nitride is a silicon/nitrogen material forwhich x/y is greater than ¾. Preferably, second insulating layer 62comprises a silicon oxide. In an exemplary embodiment of the invention,multi-layer stack 58 has a total thickness that is no greater than about25 nm. A control gate 66 overlies the upper oxide layer 62 of the chargetrapping stack 58 of each gate stack 52, 54. The control gate may beformed of polycrystalline silicon and may be doped with an N-typeimpurity.

As illustrated in FIG. 2, the first and second gate stacks 52, 54 areseparated by a trench 68 that has been formed in substrate 56 using gatestacks 52, 54 as etch masks. The trench 68 has a depth, indicated bydouble-headed arrow 72, in the range of about 100 to about 800 angstromsas measured from a surface 70 of substrate 56. An impurity doped region74, otherwise known as a bit line region, is disposed underlying trench68. In this regard, during programming of cell 52, secondary electronsresulting from secondary impact ionization are blocked by trench 68 andimpurity doped region 74 and are not able to diffuse to cell 54.Similarly, during programming of cell 54, secondary electrons resultingfrom secondary impact ionization are blocked by trench 68 and impuritydoped region 74 and are not able to diffuse to cell 52.

A conductor 76 is disposed within trench 68 and at least partially fillstrench 68. Conductor 76 comprises any suitable conductive material suchas, for example, tungsten (W), a metal silicide, polycrystallinesilicon, and the like. In a preferred embodiment, conductor 76 has athickness of at least 10 nm but does not have a thickness such that theconductor 76 and charge trapping layer 60 of multi-stack 58 are inelectrical and/or physical contact.

FIG. 3 is a top view of a portion of memory device 50. As illustrated, aplurality 51 of first gate stacks 52 is formed in a linear manner alonga length, designated by double arrow 78, of substrate 56. A plurality 53of second gate stacks 54 also are formed in a linear manner parallel tothe plurality of first gate stacks 52. Bit line 74 is disposed betweenthe plurality 51 of first gate stacks 52 and the plurality 53 of secondgate stacks 54 and extends along length 78 of substrate 56. Conductor 76is disposed within trench 68 and extends along length 78. Because of itsconductive nature, conductor 76 within trench 68 reduces the resistanceof the bit lines 74, thus increasing the speed of memory device 50.

Conductive contact 96 may be formed to conductor 76 for electricalcommunication with bit line 74. Typically, to form a contact to bit line74, an insulating material, such as a silicon oxide, may be depositedbetween gate stacks 52 and 54 and within trench 68 and a via or contactwindow is etched therethrough to the bitline. To decrease the width ofthe contact window, a second insulating material, such as a siliconnitride, then is conformally deposited over the first insultingmaterial. However, with the presence of trench 68, the second insulatingmaterial may “pinch off” the contact window, thus preventing asubsequently deposited conductive material from contacting the bitline.Because conductor 76 provides a planar surface between first gate stack52 and second gate stack 54, a conductive contact 96 can be made toconductor 76 and, hence, bit line 74 without “pinching off” challengesdue to trench 68. Accordingly, the dimensions of memory device 50 can bereduced without adversely affecting subsequent fabrication steps oroperation of device 50.

In accordance with an exemplary embodiment of the present invention,FIGS. 4-7 illustrate a method for fabricating a memory device, such asthe flash memory device 50 of FIG. 2, that can be scaled with decreaseddevice dimensions while overcoming challenges of transient programdisturb. FIGS. 4-7 illustrate various cross-sectional views of flashmemory device 50. Various steps in the manufacture of flash memorydevice 50 are well known and so, in the interest of brevity, manyconventional steps will only be mentioned briefly herein or will beomitted entirely without providing well known process details.

As illustrated in FIG. 4, the manufacture of flash memory device 50begins by providing a silicon substrate 56. As used herein, the term“silicon substrate” will be used to encompass the relatively puresilicon materials typically used in the semiconductor industry as wellas silicon admixed with other elements such as germanium, carbon, andthe like. The term “silicon substrate” also is used to encompass thesubstrate itself together with metal or insulator layers that may overlythe substrate. Silicon substrate 56 may be a bulk silicon wafer or athin layer of silicon on an insulating layer (commonly known as asilicon-on-insulator wafer or SOI wafer) that, in turn, is supported bya silicon carrier wafer.

A first insulating layer 64 and a charge trapping layer 60 of amulti-layer dielectric-charge trapping-dielectric stack 58 are formedoverlying substrate 56. Preferably insulating layer 64 is a layer ofsilicon dioxide having a thickness of about 2-10 nanometers (nm), morepreferably about 5 nm. Layer 64 can be a thermally grown layer ofsilicon dioxide or can be deposited, for example, by low pressurechemical vapor deposition (LPCVD). Charge trapping layer 60 can bedeposited, for example, to a thickness of about 3 to 20 nm by chemicalvapor deposition (CVD), plasma enhanced chemical vapor deposition(PECVD), LPCVD, or by atomic layer deposition (ALD). The silicon oxidelayers can be deposited, for example, from either atetraethylorthosilicate (TEOS) or SiH₄ (silane) source or can be grownthermally from silicon, silicon nitride, or silicon-rich siliconnitride. The silicon nitride or silicon-rich silicon nitride can bedeposited, for example, from the reaction of dichlorosilane and ammonia.

A control gate material layer 80, preferably of polycrystalline siliconor, in the alternative, metal or other conductive material, is depositedoverlying the multi-layer stack 58. The layer of polycrystalline siliconcan be deposited as an impurity doped layer, but is preferably depositedas undoped polycrystalline silicon and is subsequently impurity doped byion implantation. A layer 82 of antireflective coating material (ARC)such as silicon oxide, silicon nitride, or silicon oxynitride can bedeposited onto the surface of the polycrystalline silicon layer 80 toaid in subsequently patterning the polycrystalline silicon. Thepolycrystalline material can be deposited to a thickness of about 300angstroms to about 1200 angstroms by LPCVD by the hydrogen reduction ofsilane (SiH₄). ARC layer 82 can be deposited to a thickness of about 50nm, also by LPCVD.

In one embodiment of the invention, the ARC layer 82 can be patternedusing conventional photolithography techniques to form an opening 84. Aninsulating layer of silicon oxide or silicon nitride (not shown) can bedeposited within opening 84 and overlying ARC layer 82 and thenanisotropically etched, such as by reactive ion etching (RIE) usingCHF₃, CF₄, or SF₆ chemistry or any other suitable method, to formspacers 86 about the sidewalls of ARC layer 82.

Referring to FIG. 5, the polycrystalline silicon layer 80 and themulti-layer stack 58 are etched by, for example, plasma etching in a Clor HBr/O₂ chemistry, to form control gates 66 of a first gate stack 52and a second gate stack 54. Silicon substrate 56 is etched by, forexample, the same chemistry, to form a trench 68 within siliconsubstrate 56. As described above, trench 68 has a depth, illustrated bydouble-headed arrow 72, in the range of about 100 to about 800 angstromsas measured from a surface 70 of substrate 56.

Referring to FIG. 6, after formation of trench 68, gate stacks 52 and 54are used as an ion implantation mask to form an impurity doped region orbit line region 74 in silicon substrate 56 below trench 68. The bit lineregion 74 preferably is formed by implanting an N-type impurity dopant,preferably arsenic ions or phosphorous ions, using an ion implant energyin the range of about 2 keV to about 30 keV. Preferably, the ion implantenergy is in the range of about 3 keV to about 20 keV. The bit lineregion 74 and the trench 68 have a combined depth, illustrated bydouble-headed arrow 88, of about 400 to about 1200 angstroms. Followingthe formation of bit line region 74, the ARC layer 82 and spacers 86 canbe removed using conventional methods.

The method in accordance with an embodiment of the invention continueswith the partial filling of trench 68 with a conductive material to forma conductor 76 within trench 68 and in electrical contact with bit line74, as illustrated in FIG. 7. As described above, the conductivematerial may be any suitable conductive material such as, for example,W, metal silicide, polycrystalline silicon, and the like. In oneexemplary embodiment of the invention, the conductive material may bedeposited by CVD, PECVD, LPCVD, ALD, and the like and then etched to adesired thickness. In another exemplary embodiment of the invention,such as when forming conductor 76 from a metal silicide, the metalsilicide layer is formed by depositing a silicide-forming metal in thetrench and heating, such as by rapid thermal annealing (RTA), to causethe silicide-forming metal to react with silicon substrate 56. Thesilicide-forming metal can be, for example, cobalt, nickel, rhenium,ruthenium, or palladium. The silicide-forming metal can be deposited,for example, by sputtering. The RTA causes the formation of metalsilicide anywhere the silicide-forming metal is in contact with exposedsilicon. Any silicide-forming metal that is not in contact with exposedsilicon does not react during the RTA to form a silicide and maysubsequently be removed by wet etching in an H₂O₂/H₂SO₄ or HNO₃/HClsolution. In an exemplary embodiment of the invention, the conductor 76has a thickness, indicated by double-headed arrow 88, that is no lessthan 10 nm. In another exemplary embodiment of the invention, theconductor 76 is not so thick that it comes in electrical contact withcharge trapping layer 60 causing an electrical short therebetween. Inaddition, the conductor 76 is not so thick that it causes a breakdownbetween a subsequently-formed word line 126, discussed in more detailbelow, and the bit line 74. In a preferred embodiment of the invention,the conductor 76 has a thickness such that a surface 90 of the conductoris approximately planar with surface 70 of substrate 56.

In accordance with another exemplary embodiment of the invention, anyremaining portion of trench 68 and/or a space 120 between first gatestack 52 and second gate stack 54 that is not filled by conductor 76 maybe filled by an insulating material 118. Insulating material 118 may beany suitable insulating material such as, for example, silicon oxide,silicon nitride, or, preferably, high density plasma silicon oxide (HDP)and may be deposited as a blanket layer by CVD, LPCVD, or the like. Inone embodiment of the present invention, after deposition the insulatingmaterial 118 may be planarized to expose ARC layer 82 and spacers 86,which then may be removed by conventional methods. In another embodimentof the present invention, the insulating material 118, the ARC layer 82,and spacers 86 may be planarized to expose a surface of control gates66.

The method in accordance with one embodiment of the invention iscontinued by depositing a blanket layer 122 of polycrystalline siliconor other conductive material in electrical contact with control gates66. The blanket layer is preferably deposited as an impurity doped layerof polycrystalline silicon or can be deposited as a polycrystallinesilicon that is subsequently doped by ion implantation. Although notseen in this cross sectional view, blanket layer 122 isphotolithographically patterned and etched to form a word line 126 thatis electrically coupled to control gates 66 and that is disposedperpendicular to bit line 74. As described above, conductor 76, whichextends the length of bit line 74, reduces the resistance of bit line 74and, hence, increases the speed of memory device 50. In addition,because conductor 76 provides a planar surface between first gate stack52 and second gate stack 54, a conductive contact (not shown) can bemade through insulating layer 118 to conductor 76 and, hence, bit line74 without “pinching off” challenges due to trench 68. Accordingly, thedimensions of memory device 50 can be reduced without adverselyaffecting subsequent fabrication steps or operation of device 50.

FIGS. 8-12 illustrate a method for fabricating a memory device 100 inaccordance with another exemplary embodiment of the present invention.Flash memory device 100 is similar to flash memory device 50 of FIGS.4-7 and, accordingly, elements of FIGS. 8-12 that have the samereference numbers as elements of FIGS. 4-7 are the same elements ofFIGS. 4-7 and may be formed of any of the same materials and by any ofthe same methods as described above with reference to FIGS. 4-7. Themethod described with reference to FIGS. 8-12 begins by utilizing thesteps described above with reference to FIG. 4, which, for brevity, willnot be described again. Accordingly, after performing the stepsillustrated in FIG. 4, the polycrystalline silicon layer 80 and themulti-layer stack 58 are etched to form control gates 66 of first gatestack 52 and second gate stack 54 and to expose silicon substrate 56, asillustrated in FIG. 8.

After formation of gate stacks 52 and 54, a gate spacer material layer(not shown) is deposited conformally overlying gate stacks 52 and 54 andsubstrate 56. The gate spacer material layer may be formed of anysuitable insulating material, such as silicon oxide, silicon nitride, ora suitable polymer. The gate spacer material then is anisotropicallyetched by, for example, RIE using a CHF₃, CF₄, or SF₆ chemistry, to formgate spacers 102 about sidewalls 104 of gate stacks 52 and 54, asillustrated in FIG. 9.

Referring to FIG. 10, using the gate spacers 102 and gate stacks 52 and54 as an etch mask, the silicon substrate 56 is etched to form a trench106 in the silicon substrate. Silicon substrate 56 may be etched by, forexample, plasma etching in a C₄H₈ and O₂ chemistry and/or an HBrchemistry. In an exemplary embodiment of the invention, the trench 106has a depth, illustrated by a double-headed arrow 108, in the range ofabout 100 to about 800 angstroms as measured from surface 70 ofsubstrate 56. After formation of trench 106, gate stacks 52 and 54 andgate spacers 102 are used as an ion implantation mask to form animpurity doped region or bit line region 110 in silicon substrate 56below trench 106. The bit line region 110 preferably is formed byimplanting an N-type impurity dopant, preferably arsenic ions orphosphorous ions, using an ion plasma energy in the range of about 2 keVto about 30 keV. Preferably, the ion implant energy is in the range ofabout 3 keV to about 20 keV. In an exemplary embodiment of theinvention, the bit line region 110 and the trench 106 have a combineddepth, illustrated by double-headed arrow 112, of about 400 to 1200angstroms. By using gate spacers 102, the width of space 120 betweenmemory cells 52 and 54 can be made smaller. Accordingly, the depth 112of trench 106 and bit line region 110 can be larger than the combineddepth of trench 68 and bit line region 74 of FIG. 7 while beingfabricated using the same ion implant energy.

The method in accordance with an embodiment of the invention continueswith the at least a partial filling of trench 106 with a conductivematerial to form a conductor 114, as illustrated in FIG. 11. Conductor114 can be formed using the same materials and the same methods asdescribed above for conductor 76 of FIG. 7. In one embodiment of thepresent invention, trench 106 is partially filled by the conductor 114.In another embodiment of the present invention, trench 106 isapproximately fully filled by the conductor 114. In yet anotherembodiment of the present invention, depending on a thickness, indicatedby double-headed arrow 124, of gate spacers 102, conductor 114 maycompletely fill trench 106 and have a thickness, indicated bydouble-headed arrow 116, as measured from the bottom of trench 106, suchthat the conductor 114 partially fills space 120 between sidewallspacers 102. If conductor 114 has a thickness 116 such that a surface128 of conductor 114 extends to or past charge trapping layer 60, gatespacers 102 have a thickness 124 such that conductor 114 is electricallyisolated from charge trapping layer 60. The presence of gate spacers 102permits conductor 114 to have a greater thickness than that of conductor76 of FIG. 7, which thus decreases the resistance of bit line 110 andincreases the speed of memory device 100.

Referring to FIG. 12, in accordance with another exemplary embodiment ofthe invention, any remaining portion of trench 106 and/or space 120 thatis not filled by conductor 114 may be filled by an insulating material118. Insulating material 118 may be any suitable insulating materialsuch as, for example, silicon oxide, silicon nitride, or, preferably,high density plasma silicon oxide (HDP) and may be deposited as ablanket layer by CVD, LPCVD, or the like. In one embodiment of thepresent invention, after deposition the insulating material 118 may beplanarized to expose ARC layer 82 and spacers 86, which then may beremoved by conventional methods. In another embodiment of the presentinvention, the insulating material 118, the ARC layer 82, and spacers 86may be planarized to expose a surface of control gates 66.

The method in accordance with one embodiment of the invention iscontinued by depositing a blanket layer 122 of polycrystalline siliconor other conductive material in electrical contact with control gates66. The blanket layer is preferably deposited as an impurity doped layerof polycrystalline silicon or can be deposited as a polycrystallinesilicon that is subsequently doped by ion implantation. Although notseen in this cross sectional view, blanket layer 122 isphotolithographically patterned and etched to form a word line 126electrically coupled to control gates 66. As described above, gatespacers 102 electrically isolate charge trapping layer 60 of chargetrapping stack 58 from conductor 114. In addition, it will beappreciated that gate spacers 102 have a thickness 124 sufficient toprevent breakdown between the word line 126 and the control gates 66.

FIGS. 13-17 illustrate a method for fabricating a memory device 150 inaccordance with another exemplary embodiment of the present invention.Flash memory device 150 is similar to flash memory device 50 of FIGS.4-7 and, accordingly, elements of FIGS. 13-17 that have the samereference numbers as elements of FIGS. 4-7 are the same elements ofFIGS. 4-7 and may be formed of any of the same materials and by any ofthe same methods as described above with reference to FIGS. 4-7. Themethod described with reference to FIGS. 13-17 begins by utilizing thesteps described above with reference to FIG. 4, which, for brevity, willnot be described again. Accordingly, after performing the stepsillustrated in FIG. 4, the polycrystalline silicon layer 80 and themulti-layer stack 58 are etched to form control gates 66 of first gatestack 52 and second gate stack 54 and to expose silicon substrate 56.Gate stacks 52 and 54 are used as an ion implantation mask to form afirst, shallow impurity doped region or bit line region 152 in siliconsubstrate 56. First bit line region 152 preferably is formed byimplanting an N-type impurity dopant, preferably arsenic ions orphosphorous ions, using an ion implant energy in the range of about 2keV to about 30 keV. Preferably, the ion implant energy is in the rangeof about 3 keV to about 15 keV.

The method in accordance with one embodiment of the invention iscontinued by depositing a gate spacer material layer (not shown)conformally overlying gate stacks 52 and 54 and substrate 56, asillustrated in FIG. 14. The gate spacer material then is anisotropicallyetched by, for example, RIE using a CHF₃, CF₄, or SF₆ chemistry, to formgate spacers 102 about the sidewalls 104 of gate stacks 52 and 54. Usingthe gate spacers 102 and gate stacks 52 and 54 as an etch mask, thesilicon substrate 56 is etched by, for example, plasma etching in a C₄F₈and O₂ chemistry, to form a trench 156 in the silicon substrate. Trench156 has a depth, illustrated by double-headed arrow 158, in the range ofabout 100 to about 800 angstroms as measured from surface 70 ofsubstrate 56. Trench 156 can be shallower or deeper than bit line region152.

Referring to FIG. 15, after formation of trench 156, gate stacks 52 and54 and gate spacers 102 are used as an ion implantation mask to form asecond, deeper impurity doped region or bit line region 160 in siliconsubstrate 56 below trench 106. The second bit line region 160 preferablyis formed by implanting an N-type impurity dopant, preferably arsenicions or phosphorous ions, using an ion implant energy in the range ofabout 5 keV to about 40 keV. The trench 156 and second bit line region160 preferably have a combined depth, illustrated by double-headed arrow162, of about 400 to 1200 angstroms. Because of the presence of trench156, second bit line region 160 does not have to be formed at as high animplant energy to block the diffusion of secondary electrons generatedduring programming as would be required without the presence of trench156.

The method in accordance with an embodiment of the invention continueswith the at least a partial filling of trench 156 with a conductivematerial to form a conductor 164, as illustrated in FIG. 16. Conductor164 can be formed using the same materials and the same methods asdescribed above for conductor 76 of FIG. 7. Similar to conductor 114 ofFIGS. 11 and 12, in one embodiment of the present invention, trench 156is partially filled by the conductor 164. In another embodiment of thepresent invention, trench 156 is approximately fully filled by theconductor 164. In yet another embodiment of the present invention,depending on a thickness 124 of gate spacers 102, conductor 164 maycompletely fill trench 156 and have a thickness, indicated bydouble-headed arrow 166, as measured from the bottom of trench 156, suchthat the conductor 164 partially fills space 120 between sidewallspacers 102, as illustrated in FIG. 16. If conductor 164 has a thickness166 such that a surface 168 of conductor 164 extends to or past chargetrapping layer 60, gate spacers 102 have a thickness 124 such thatconductor 164 is electrically isolated from charge trapping layer 60. Asdescribed above, the presence of gate spacers 102 permits conductor 164to have a greater thickness than that of conductor 76 of FIG. 7, whichthus decreases the resistance of bit lines 152 and 160 and increases thespeed of memory device 150.

Referring to FIG. 17, in accordance with an exemplary embodiment of theinvention, any remaining portion of trench 156 and/or space 120 that isnot filled by conductor 164 may be filled by insulating material 118. Inone embodiment of the present invention, after deposition the insulatingmaterial 118 may be planarized to expose ARC layer 82 and spacers 86,which then may be removed by conventional methods. In another embodimentof the present invention, the insulating material 118, the ARC layer 82,and spacers 86 may be planarized to expose a surface of control gates66.

The method in accordance with one embodiment of the invention iscontinued by depositing blanket layer 122 of polycrystalline silicon orother conductive material in electrical contact with control gates 66.Although not seen in this cross sectional view, blanket layer 122 isphotolithographically patterned and etched to form word line 126electrically coupled to control gates 66. As described above, gatespacers 102 electrically isolate charge trapping layer 60 of chargetrapping stack 58 from conductor 164. In addition, it will beappreciated that gate spacers 102 have a thickness 124 sufficient toprevent breakdown between the word line 126 and the control gates 66.

FIGS. 18-21 illustrate a method for fabricating a memory device 200 inaccordance with another exemplary embodiment of the present invention.Flash memory device 200 is similar to flash memory device 50 of FIGS.4-7 and, accordingly, elements of FIGS. 18-21 that have the samereference numbers as elements of FIGS. 4-7 are the same elements ofFIGS. 4-7 and may be formed of any of the same materials and by any ofthe same methods as described above with reference to FIGS. 4-7. Themethod described with reference to FIGS. 18-21 begins by utilizing thesteps described above with reference to FIG. 4, which, for brevity, willnot be described again. Accordingly, after performing the stepsillustrated in FIG. 4, the polycrystalline silicon layer 80 and themulti-layer stack 58 are etched to form control gates 66 of first gatestack 52 and second gate stack 54 and to expose silicon substrate 56.Gate stacks 52 and 54 are used as an ion implantation mask to form afirst, shallow impurity doped region or bit line region 202 in siliconsubstrate 56. First bit line region 202 preferably is formed byimplanting an N-type impurity dopant, preferably arsenic ions orphosphorous ions, using an ion plasma energy in the range of about 2 keVto about 30 keV. Preferably, the ion implant energy is in the range ofabout 3 keV to about 20 keV.

Referring to FIG. 19, after formation of first bit line region 202, agate spacer material layer (not shown) is deposited conformallyoverlying gate stacks 52 and 54 and substrate 56. The gate spacermaterial then is anisotropically etched by, for example, RIE using aCHF₃, CF₄, or SF₆ chemistry, to form gate spacers 102 about thesidewalls 104 of gate stacks 52 and 54. Gate stacks 52 and 54 and gatespacers 102 are used as an ion implantation mask to form a second,deeper impurity doped region or bit line region 206 in silicon substrate56. The second bit line region 206 preferably is formed by implanting anN-type impurity dopant, preferably arsenic ions or phosphorous ions,using an ion plasma energy in the range of about 10 keV to about 50 keV.The first bit line region 202 and the second bit line region 206preferably have a combined depth, illustrated by double-headed arrow208, as measured from surface 70 of substrate 56 of about 400 to 1200angstroms.

The method in accordance with an embodiment of the invention continueswith the at least a partial filling of space 120 with a conductivematerial to form a conductor 210, as illustrated in FIG. 20. Conductor210 can be formed using the same materials and the same methods asdescribed above for conductor 76 of FIG. 7. Conductor 210 may have athickness, indicated by double-headed arrow 212, as measured fromsubstrate 56, such that the conductor 210 partially fills space 120between sidewall spacers 102. If conductor 164 has a thickness 212 suchthat a surface 214 of conductor 164 extends to or past charge trappinglayer 60, gate spacers 102 have a thickness 124 such that conductor 210is electrically isolated from charge trapping layer 60. As describedabove, the presence of gate spacers 102 permits conductor 210 to have agreater thickness than that of conductor 76 of FIG. 7, which thusdecreases the resistance of bit lines 202 and 206 and increases thespeed of memory device 200.

Referring to FIG. 21, in accordance with an exemplary embodiment of theinvention, any remaining portion of space 120 that is not filled byconductor 210 may be filled by insulating material 118. In oneembodiment of the present invention, after deposition the insulatingmaterial 118 may be planarized to expose ARC layer 82 and spacers 86,which then may be removed by conventional methods. In another embodimentof the present invention, the insulating material 118, the ARC layer 82,and spacers 86 may be planarized to expose a surface of control gates66.

The method in accordance with one embodiment of the invention iscontinued by depositing blanket layer 122 of polycrystalline silicon orother conductive material in electrical contact with control gates 66.Although not seen in this cross sectional view, blanket layer 122 isphotolithographically patterned and etched to form word line 126electrically coupled to control gates 66. As described above, gatespacers 102 electrically isolate charge trapping layer 60 of chargetrapping stack 58 from conductor 210. In addition, it will beappreciated that gate spacers 102 have a thickness 124 sufficient toprevent breakdown between the word line 126 and the control gates 66.

Accordingly, flash memory devices and methods for fabricating flashmemory devices have been provided. The devices and methods provide forthe scaling of the memory devices while overcoming transient programdisturb challenges. In addition, the methods provide for the reductionin bit line resistance and, hence, a corresponding increase in speed ofthe memory device. While at least one exemplary embodiment has beenpresented in the foregoing detailed description of the invention, itshould be appreciated that a vast number of variations exist. It shouldalso be appreciated that the exemplary embodiment or exemplaryembodiments are only examples, and are not intended to limit the scope,applicability, or configuration of the invention in any way. Rather, theforegoing detailed description will provide those skilled in the artwith a convenient road map for implementing an exemplary embodiment ofthe invention, it being understood that various changes may be made inthe function and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

1. A method for fabricating a memory device, the method comprising thesteps of: fabricating a first gate stack and a second gate stackoverlying a substrate; etching a trench into the substrate between thefirst gate stack and the second gate stack; forming a first impuritydoped region within the substrate underlying the trench; and filling thetrench at least partially with a conductive material.
 2. The method ofclaim 1, wherein the step of filling the trench at least partially witha conductive material comprises the step of filling the trench at leastpartially with tungsten, a metal silicide or polycrystalline silicon. 3.The method of claim 1, further comprises the step of forming gatespacers about sidewalls of first gate stack and second gate stack beforethe step of etching a trench into the substrate.
 4. The method of claim3, wherein the step of filling the trench at least partially with aconductive material comprises the step of filling the trench andpartially filling a space between the first gate stack and the secondgate stack with the conductive material.
 5. The method of claim 4,wherein the step of fabricating a first gate stack and a second gatestack overlying a substrate comprises the steps of: forming a chargetrapping layer overlying the substrate; depositing a control gatematerial layer overlying the substrate; and etching the control gatematerial layer and the charge trapping layer to expose the siliconsubstrate.
 6. The method of claim 5, wherein the step of forming gatespacers comprises the step of forming gate spacers having a thicknesssufficient to electrically isolate the conductive material from thecharge trapping layer.
 7. The method of claim 5, further comprising thesteps of: depositing an insulating layer overlying the conductivematerial; and fabricating a word line overlying the insulating layer andin electrical contact with first gate stack and the second gate stack.8. The method of claim 7, wherein the step of forming gate spacerscomprises the step of forming gate spacers having a thickness sufficientto prevent breakdown between the word line and the control gate materiallayer of the first gate stack and the second gate stack.
 9. The methodof claim 3, further comprising the step of forming a second impuritydoped region between the first gate stack and the second gate stackbefore the step of etching a trench into the substrate.
 10. A method forfabricating a dual bit memory device, the method comprising the stepsof: fabricating a charge trapping stack overlying a substrate; forming acontrol gate material overlying the charge trapping stack; etching thecontrol gate material and the charge trapping stack to the substrate toform a plurality of first gate stacks and a plurality of second gatestacks, wherein the plurality of first gate stacks are disposed in alinear manner along a length of the substrate and the plurality ofsecond gate stacks are disposed in a linear manner along the length ofthe substrate parallel to the plurality of first gate stacks; etchingthe substrate to form a trench that extends between the plurality offirst gate stacks and the plurality of second gate stacks along thelength of the substrate; implanting a first impurity dopant within thesubstrate underlying the trench to form a first bit line region; andforming a conductor within the trench, wherein the conductor extendsalong the length of the substrate.
 11. The method of claim 10, whereinthe step of forming a conductor comprises the step of forming theconductor from tungsten, a metal silicide or polycrystalline silicon.12. The method of claim 10, further comprising the step of forming gatespacers about sidewalls of each of the plurality of first gate stacksand of each of the plurality of second gate stacks before the step ofetching the substrate to form a trench.
 13. The method of claim 12,wherein the step of forming a conductor with the trench comprises thestep of filling the trench and partially filling a space between thefirst gate stack and the second gate stack with a conductive material.14. The method of claim 13, wherein the step of fabricating a chargetrapping stack overlying a substrate comprises the steps of: forming afirst insulating layer overlying the substrate; fabricating a chargetrapping layer overlying the first insulating layer; forming a secondinsulating layer overlying the charge trapping layer.
 15. The method ofclaim 14, wherein the step of forming gate spacers comprises the step offorming gate spacers having a thickness sufficient to electricallyisolate the conductor from the charge trapping layer.
 16. The method ofclaim 14, further comprising the steps of: depositing an insulatinglayer overlying the conductor; and fabricating a word line overlying theinsulating layer and disposed perpendicular to first bit line region.17. The method of claim 16, wherein the step of forming gate spacerscomprises the step of forming gate spacers having a thickness sufficientto prevent breakdown between the word line and the control gate materiallayer.
 18. The method of claim 12, further comprising the step ofimplanting a second impurity dopant within the substrate before the stepof etching the substrate to form a trench.
 19. A method for fabricatinga memory device, the method comprising the steps of: fabricating a firstgate stack and a second gate stack overlying a substrate; forming afirst impurity doped region in the substrate between the first gatestack and the second gate stack; fabricating gate spacers aboutsidewalls of the first gate stack and the second gate stack; forming asecond impurity doped region in the substrate between the first gatestack and the second gate stack using the gate spacers as an implantmask, wherein the second impurity doped region is deeper than the firstimpurity doped region; and filling a space between the first gate stackand the second gate stack with a conductive material.
 20. The method ofclaim 19, wherein the step of filling a space between the first gatestack and the second gate stack with a conductive material comprises thesteps of filling the space with tungsten, a metal silicide orpolycrystalline silicon.
 21. A memory device comprising: a plurality offirst gate stacks and a plurality of second gate stacks overlying asubstrate, wherein the plurality of first gate stacks are disposed in alinear manner along a length of the substrate and the plurality ofsecond gate stacks are disposed in a linear manner along the length ofthe substrate parallel to the plurality of first gate stacks; aconductor disposed within a trench in the substrate between theplurality of first gate stacks and the plurality of second gate stacks,wherein the conductor and the trench extend along the length of thesubstrate; and a first impurity doped region disposed within thesubstrate and underlying the trench and the conductor, wherein the firstimpurity doped region extends along the length of substrate.
 22. Thememory device of claim 21, wherein the conductor comprises tungsten, ametal silicide or a polycrystalline silicon.
 23. The memory device ofclaim 21, further comprising gate spacers disposed along sidewalls ofeach of the plurality of first gate stacks and each of the plurality ofsecond gate stacks.
 24. The memory device of claim 23, wherein each ofthe plurality of first gate stacks and the plurality of second gatestacks comprises a charge trapping layer overlying the substrate and acontrol gate overlying the charge trapping layer.
 25. The memory deviceof claim 24, wherein the gate spacers have a thickness sufficient toelectrically isolate the conductor from the charge trapping layers ofeach of the plurality of first gate stacks and a plurality of secondgate stacks.
 26. The memory device of claim 24, further comprising aninsulating material overlying the conductor and between the plurality offirst gate stacks and the plurality of second gate stacks and a wordline disposed overlying the insulating material and perpendicular to thefirst impurity doped region.
 27. The memory device of claim 26, whereinthe gate spacers have a thickness sufficient to prevent breakdownbetween the word line and the control gate of each of the plurality offirst gate stacks and the plurality of second gate stacks.
 28. Thememory device of claim 21, further comprising a second impurity dopedregion disposed within the substrate, wherein the second impurity dopedregion extends along the length of substrate and is shallower than thefirst impurity doped region.
 29. A memory device comprising: a pluralityof first gate stacks and a plurality of second gate stacks overlying asubstrate, wherein the plurality of first gate stacks are disposed in alinear manner along a length of the substrate and the plurality ofsecond gate stacks are disposed in a linear manner along the length ofthe substrate parallel to the plurality of first gate stacks; a firstimpurity doped region disposed within the substrate between theplurality of first gate stacks and the plurality of second gate stacksalong the length of substrate; a second impurity doped region disposedwithin the substrate between the plurality of first gate stacks and theplurality of second gate stacks along the length of substrate, whereinthe second impurity doped region is disposed deeper within the substratethan the first impurity doped region; and a conductor disposed overlyingthe substrate between the plurality of first gate stacks and a pluralityof second gate stacks, wherein the conductor extends along the length ofthe substrate and is in electrical contact with the first and secondimpurity doped regions.
 30. The memory device of claim 29, wherein theconductor is formed of tungsten, metal silicide, or polycrystallinesilicon.